Accurate measurement of the masses of atoms and molecules (mass-spectrometry) is one of the most efficient methods for qualitative and quantitative analysis of chemical compositions of substances. The substance under investigation is first ionised using one of a number of available ionisation methods (e.g. electron impact, discharge, laser irradiation, surface ionization, electro-spray). In time-of-flight (TOF) mass spectrometers ions are extracted from an ion source as discrete ion pulses using an electric field and, after acceleration, are directed into a flight path of the analyser. Due to the laws of motion in an electrostatic field the flight times of ions having different mass-to-charge ratios (but the same average energy) is proportional to the square root of mass-to-charge ratio. Thus, ions are separated into discrete packets according to their mass-to-charge ratios and can be registered sequentially by a detector to form a mass spectrum.
The higher the total flight times of ions in the TOF analyser, the better the resolving power of mass analysis. For this reason several types of TOF mass analyser with increased flight path due to multiple reflections have been developed. Increasing the flight time of ions, while keeping the size of the ion packets sufficiently small, is a difficult task because of a spread of initial positions of ions inside the source, which results in a deviation of kinetic energy from an average value (energy spread) and due to spread of initial ion velocities which results in so-called turn-around time and a lateral angular spread of the beam. In order to obtain a mass spectrum in a wide mass range with high sensitivity it is desirable to satisfy several conflicting conditions at the same time; that is, to: 1) avoid looping of the beam trajectory; 2) ensure lateral stability of the ion beam and; 3) obtain space-energy focusing at the surface of the detector with minimum aberrations. Because of this, the development of multi-reflecting TOF (mTOF) system has involved optimisation of the ion optics in order to increase acceptance; that is the volume of phase space which can be accepted by the system. So far, the problem has been addressed in the main using sophisticated optimisation software, although each particular design has inherent advantages and disadvantages.
Although the acceptance of existing multi-reflecting TOF systems is suitable for many ion sources which employ cooling using buffer gas and high extraction fields, such systems are not well suited directly to accept ions having wide energy and angular spread as produced, for example by a matrix-assisted laser desorption/ionization (MALDI) ion source.